Its a great idea, please post pictures if you go ahead with it I would like to see how it looks inside the rearview mirror.
There is another crazy option using the on-board cameras we rc plane guys use.
These have a tiny transmitter which could easily transmit to any location in the car (where the recorder is housed), however there is the question of longevity of the transmitter on long trips of several hours as they do get warm.
http://www.fpvhobby.com/95-8ghz-mini-combset.html
I should add, some of the power outputs on some frequencies are illegal in the UK for civilian use.